This invention relates generally to packet-switching systems, such as asynchronous transfer mode (ATM) systems, and specifically to transmission-delay variations in such systems.
Today""s business communications environment consists of two separate network infrastructures: a voice network (such as a private branch exchange (PBX)) characterized by real-time, high-reliability, constant bit-rate (CBR) connections; and a data network (such as a packet network) characterized by high-bandwidth variable bit-rate (VBR) connections. Business needs for simplified maintenance, management, and access to information on diverse networks are forcing the convergence of these networks along with a new class of real-time multimedia networks. Asynchronous transfer mode (ATM) provides a single infrastructure that cost-effectively and flexibly handles both switching and transmission for the traffic types mentioned above (voice, video, and data) for both local-area networks and wide-area networks. The evolving network convergence requires the adaptation of the legacy PBX voice traffic to ATM. Voice telephony over ATM (VTOA) specifications allow adaptation of compressed or uncompressed voice pulse-code modulated (PCM) data streams into streams (virtual circuits) of CBR cells.
An ATM cell, regardless of the traffic it carries, is a packet 53 octets long: 48 octets of payload attached to a 5-octet header. The header contains addressing and management information used to direct the cell from source to destination and to ensure that the negotiated aspects of the traffic-flow through the ATM network are met. CBR traffic is assembled into cell payloads using ATM Adaptation Layer 1 (AAL1). The AAL1 cell constructor layer uses the first octet of the payload for its header and the remaining 47 octets to carry CBR information. Once every eight cells, the AAL1 structured data transfer (SDT) cell constructor layer introduces a one-octet pointer into the CBR payload. This pointer designates traffic-block boundaries. It is used at the receiving end to generate framing signals for devices that convert the ATM traffic into T1 or E1 (telephony trunk) traffic. ATM cell construction is then completed by attaching the ATM header to the payload.
Known existing devices that convert the ATM traffic into T1 or E1 traffic do not synchronize (align) the received blocks of traffic with the appropriate bits on a T1, E1, or TDM bus that is synchronized to another source (e.g., the destination synchronization source).
This invention is directed to solving these and other problems and disadvantages of the prior art. According to the invention, the received blocks of traffic are synchronized with the destination synchronization source. The synchronization is advantageously effected using the SDT block-boundary pointer. Generally according to one aspect of the invention, synchronous information received by a receiver over an asynchronous communications link is synchronized with a synchronous communications medium at the receiver as follows. The receiver asynchronously receives a stream of information, including the synchronous information, and an indication (e.g., the SDT block-boundary pointer) of where in the stream of information occurred a boundary between blocks (e.g., time-division multiplex (TDM) frames) of the stream of information, and transmits the received stream on the medium (e.g., a TDM bus). The receiver also detects where boundaries between blocks (e.g., between TDM frames) for information occur on the medium, and uses the indication (the pointer) to synchronize transmissions from the receiver on the medium of information at the boundary in the stream of information with occurrence of a boundary between blocks on the medium. Illustratively, both the indication and the boundaries between blocks for information on the medium represent boundaries between TDM frames of received traffic, so that the receiver aligns TDM frames of received traffic with TDM frames on the medium. If the receiver includes a time-slot interchanger (TSI) for reordering the time slots of TDM frames of received traffic, the TSI is reset by each indication, so that TDM frames generated by the TSI for transmission on the medium align with the TDM frames of received traffic.
According to another aspect of the invention, a first synchronous communications medium (e.g., a TDM bus) at a receiver of an asynchronous communications link (e.g., an ATM link) is synchronized with a second synchronous communications medium (e.g., a TDM bus) at a transmitter of the asynchronous communications link as follows. The transmitter detects a boundary (e.g., a framing signal) between blocks (e.g., TDM frames) of information in a stream of information that it is receiving from the second medium, and transmits the information asynchronously from the transmitter to the receiver with a first indication (e.g., an SDT pointer) of where in the stream of information the boundary occurred. The information and the first indication are received by the receiver, which transmits the information on the first medium. The receiver also detects where boundaries (e.g., framing signals) between blocks (e.g., TDM frames) for information occur on the first medium, and in response to receipt of the first indication the receiver synchronizes transmission on the first medium of the information at the boundary indicated by the first indication with occurrence of a boundary between blocks on the first medium. Again, if the receiver includes a TSI, the TSI is preferably reset to the beginning of frame generation by each received first indication.
The invention includes both a method of as well as a corresponding apparatus and a computer readable medium that contains software which, when executed in a computer, causes the computer to perform the method. The apparatus preferably includes an effectorxe2x80x94any entity that effects the corresponding step, unlike a meansxe2x80x94for each method step.
These and other features and advantages of the present invention will become more apparent from the following description of an illustrative embodiment of the invention considered together with the drawing.